1. Field of the Invention
The present invention relates to a driving circuit for a so-called brushless motor in which, for example, a permanent magnet is used as a rotor, and a rotational alternating magnetic field is generated as a field, and also to a method of controlling such a circuit, and more particularly to a driving circuit for driving a brushless motor without requiring a rotational position sensor for a rotor.
2. Description of the Related Art
As a method of regulating a cooling ability of a refrigerating air conditioning apparatus by driving a compressor and the like at a variable speed, a method is generally employed in which an electric motor serving as a driving source for the compressor is driven at a variable speed. Particularly, it is known that a brushless motor in which an armature winding is wound on the stator, and a permanent magnet is mounted on the rotor operates efficiently. In a brushless motor, it is necessary to switch over magnetic poles for the field in accordance with the rotational position of the rotor, and therefore a sensor for detecting the rotational position is attached to the motor. In the case where a brushless motor is used in a hermetic compressor or the like, however, it is difficult to attach a rotational position sensor to the motor because the electric motor itself is closed and the interior of the electric motor has a high temperature. As a result, a driving circuit for the brushless motor has a somewhat complicated configuration.
As a driving circuit for a brushless motor in which a rotational position sensor is not used, conventionally, a circuit described in pages 241 to 243 of "Introduction to Power Electronics (revised second edition), 1991" edited by Yamamura and written by Ohno is used.
FIG. 14 is a diagram showing a conventional brushless motor driving circuit described in the above-specified literature. Hereinafter, with reference to the figure, the configuration of the conventional brushless motor driving circuit will be described.
In FIG. 14, 18 denotes a brushless motor driven in a 120-degree current-supplying system, 19 denotes a three-phase bridge circuit connected to three-phase terminals of the brushless motor 18, 20 denotes a voltage phase detecting circuit which detects an induced voltage of the brushless motor 18, 21 denotes a control circuit which performs the generation of a timing pulse for driving the brushless motor 18 in the 120-degree current-supplying system with respect to a preset number of rotation, and other operations, and 22 denotes a voltage doubler rectifying circuit which is connected between an AC power source 23 and the three-phase bridge circuit 19.
Referring to the figure, a variable rate limiting circuit 1211 is used for slowly accelerating the rotational speed from a very slow speed condition at the starting. A judging circuit 1212 judges, when the speed has been increased to some extent, whether it is necessary to change the control method to a method by a magnetic pole position detecting circuit, or not. A V/f converting circuit 1213 generates a pulse width modulation signal which becomes a three-phase AC voltage that is substantially proportional to the rotation speed. Selector switches 1214a to 1214c select outputs of the control circuit 21 and the V/f converting circuit 1213 in accordance with an output of the judging circuit 1212.
Next, with reference to FIG. 15, the principle of the magnetic pole position detection by the brushless motor driving circuit will be described.
FIGS. 15(a) to 15(f) are diagrams illustrating the principle of the magnetic pole position detection by the brushless motor driving circuit. FIG. 15(a) shows waveforms of the u, v, and w lines of an induced voltage of the brushless motor. FIG. 15(b) shows waveforms of line currents of the u, v, and w lines caused by a driving voltage of the brushless motor 18. FIG. 15(c) shows a waveform of a terminal voltage 1201 of the u line output from a first-order lag filter 20a (see FIG. 14). FIG. 15(d) shows a waveform of an output voltage 1202 of a comparator 20b into which the u-line terminal voltage 1201 is input (see FIG. 14). FIG. 15(e) shows a waveform of an integral voltage obtained by integrating the comparator output voltage 1202. FIG. 15(f) shows a waveform of an output voltage of a comparator (included in the control circuit 21) into which the integral voltage is input. The u-line terminal voltage is simply referred to also as the u-line voltage. The other line voltages are referred to in the same way.
In the brushless motor driving circuit, a line current is supplied to the armature winding of the brushless motor 18 only in a phase angle of 120 degrees by the three-phase bridge circuit 19, and the current is not supplied in a phase angle of 60 degrees. In the non-current-supplying period in which the current is not supplied, a voltage induced in the armature winding is detected by the voltage phase detecting circuit 20. In FIG. 15(c), the non-current-supplying period in which the u-line current does not flow is designated by .theta.u. As shown in the figure, in the non-current-supplying period .theta.u, only the u-line induced voltage appears. As shown in FIG. 15(c), a high frequency voltage caused by PWM is smoothed by the first-order lag filter 20a.
As shown in FIG. 15(b), the line current is an AC current of a square wave at a phase angle of about 120 degrees, and its fundamental wave flows so as to be in line with the induced voltage of each phase. Because the brushless motor is originally a synchronous motor, the frequency of the voltage is proportional to the number of rotation. The voltage phase detecting circuit 20 is configured so as to detect a timing when the induced voltage of each line becomes zero. For three phases, such a timing appears 6 times in one cycle. The number of rotation can be detected by measuring the intervals between respective timings. By using this, a feedback loop is configured, and the number of rotation is controlled by using the output of the number of rotation controller as a voltage instruction. Usually, this control is performed by using a microcomputer. In FIG. 15(c), the timing of the zero cross is-designated by .theta..sub.0.
Specifically, in the brushless motor driving circuit, the current supply is sequentially performed in respective phases for every 120 degrees on the three-phase terminals of the brushless motor 18. In contrast, by using the 60-degree period as a non-current-supplying period, the induced voltage of the brushless motor 18 is detected. The field magnetic poles are switched over at a zero-cross timing (in FIG. 15(f), designated by .theta.'.sub.0) of a waveform which is obtained by delaying the induced voltage waveform by 90 degrees.
In the above-described configuration of the conventional brushless motor driving circuit, the induced voltage is a voltage proportional to the rotation speed. At the starting of the motor, therefore, the induced voltage has a very small value. In addition, the terminal voltage is subjected to pulse width modulation, and hence a low-pass filter (the first-order lag filter 20a) for removing the pulse width modulation signal is used. As shown in FIG. 15(c), the induced voltage which is actually used has a further reduced amplitude, so that it is very difficult to detect the induced voltage. Accordingly, the rotational phase cannot be substantially detected. At the starting, therefore, it is impossible to drive the brushless motor by using the above-mentioned induced voltage waveform.
To comply with this, a technique is employed in which, at the starting, the V/f converting circuit 1213 is used in the same manner as the case of an induction motor and the like, so as to perform a V/f control which is a control for maintaining a uniform relationship between a voltage and a frequency. Thereafter, at a timing when the induced voltage can be detected, the control is switched to the above-described control using the induced voltage waveform, by means of the judging circuit 1212 and the selector switches 1214a to 1214c.
At the switching from the starting control to the normal control, an excessive current may flow because of a delay of the control or the deviation of parameters. For this reason, the technique has problems such as that it is necessary to use large-size power transistors for motor driving, and that there is a possibility that the permanent magnet is demagnetized by the excessive current.
There exists another problem in that, in the case where low-speed rotation in which the induced voltage cannot be detected is to be maintained, it is difficult to control the number of rotation by the starting control method described above.
Even if the induced voltage in the low speed rotation is tried to be detected by increasing the accuracy of the detecting circuit, the time width in which the induced voltage superimposed on a pulse width modulation signal can be detected is very narrow because a low voltage is applied at the start and in a low speed rotation and the ON duty of the pulse width modulation is small. Such a control can be realized by a microcomputer. There is a time lag between the ON timing instruction output from the microcomputer and the actual ON timing of the switching element. If the ON time width is narrow, therefore, an erroneous detection timing may occur. Accordingly, there exists a further problem in that it is difficult to control the low speed rotation with good accuracy.